Hi,
If these birds are all in one flock in the same cage, this type of plucking usually happens due to over crowding or territorial disputes or boredom. If you have nests in the cage, they need to be removed. You should not have nests in a cage that houses more than one pair of birds. Give them some fun, colorful toys, swings and ladders instead. If you have separate pairs that are breeding and have a nest, some plucking is done to line the nest. You have to be careful not to breed your birds too often. Two or three clutches a year is all they should have, with the nest remove between each clutch and the parents rested for 4 to 6 months. Birds that are allowed to breed too often get tired and one parent – usually the female – may tire of raising chicks. If one of a pair is not interested in mating, the other bird may pluck it. An inadequate diet can cause plucking. Finches need a balanced diet, just like parrots. A seed diet is not a balanced diet. They need a finch granule or pellets, fresh greens, chopped veggies – a foraging pellet like Nutri-Berries can be offered if you crumble them up some. This gives them some whole seeds, but the seeds are hulled and coated with a nutritionally balanced binder. And sometimes birds simply do not get along, and this can result in plucking. As for medicine, no, nothing topical or internal is going to stop plucking unless it is caused by a skin or other infection. Generally identifying the cause and changing or removing it will help.
Thank you for asking Lafeber,
Brenda